Over the years, many attempts have been made to eliminate unwanted or harmful sounds, i.e., noise. The most used technique is passive noise cancellation, which attempts to eliminate noise by muffling the noise with dampers. Passive noise control is often performed with insulation, ceiling tiles, and mufflers. Unfortunately, passive noise control systems can be bulky and work best on middle and high frequency sounds.
An attractive alternative to passive noise cancellation is active noise cancellation (“ANC”). Active noise cancellation is sound field modification by electro-acoustical means, generally by generating acoustical signals that are out of phase with the noise. In essence, active noise cancellation systems attempt to generate, electronically, a sound field that is the mirror image of the noise to be cancelled. Research into active noise cancellation began in the 1930's, with the earliest patent on active noise cancellation being granted to Lueg (U.S. Pat. No. 2,043,416) in 1936. Research continued into the 1950's with Olson and May developing an electronic sound absorber that provided a feedback mechanism for attenuating low frequency noise near a microphone. H. F. Olsen and E. G. May, “Electronic Sound Absorber,” J. Acoust. Soc. Am. 25, 1130-1136 (1953). Unfortunately, the Olson and May electronic sound absorber was unstable at higher frequencies.
Within the last 30 years, digital signal processing and advances in control theory have fed increased interest and research into active noise cancellation. This research has brought to market commercially viable active noise cancellation systems. Active noise cancellation systems are found in higher-end headphones, vehicles, and HVAC systems.
Vehicles provide a convenient example of the current use of active noise cancellation in enclosed spaces. In order to achieve active noise cancellation in vehicles, error sensors, i.e., acoustic sensors or microphones, are often placed in close proximity to the operator's head in order to detect the three-dimensional sound waves, or noise, to which the operator of the vehicle is subjected. Unfortunately, acoustic sensors located in this manner often interfere with the operator's vision, flexibility, and comfort. In addition, such acoustic sensor placement tends to provide only localized control, rather than global control of unwanted noise.
Most active noise cancellation systems focus on reducing noise by minimizing the squared acoustic pressure (“SP”). However, research by Sommerfeldt at Penn State University showed that minimizing acoustic energy density (“ED”) has advantages over minimizing SP. Acoustic energy density looks at both the pressure of the acoustic wave and its velocity. J. W. Parkins, S. D. Sommerfeldt, and J. Tichy, “Narrowband and Broadband Active Control in an Enclosure Using the Acoustic Energy Density,” J. Acoust. Soc. Am. 108, 192-203 (2000). Control of ED also has the benefit over SP in that there is less sensitivity to error sensor placement within an enclosed sound field. Using SP techniques in an enclosed sound field, there are nodal planes that exist in the three orthogonal directions; whereas, using ED, there are merely nodal lines that exist at the intersection of two orthogonal nodal planes of pressure. Therefore, for a given placement of the sensor, there is a much higher probability of the sensor being placed away from nodes. Also, ED provides more global attenuation of the noise than SP.
ED depends on acoustic particle velocity, as well as acoustic pressure. Because particle velocity is a three-dimensional quantity, most existing ED ANC systems utilize a three-dimensional energy density sensor having six acoustic sensors, with two in each of the three orthogonal directions. Each pair of acoustic sensors provides signals to a control system to yield the particle velocity component in the orthogonal direction of the pair. The vector sum of the three velocity components from the three pairs of orthogonal acoustic sensors yields particle velocity. An average of the six acoustic sensors yields acoustic pressure. A drawback of existing ED ANC systems is the additional computing power required to perform the calculations with the three-dimensional inputs forming the error signal. While certain research organizations have utilized a four-microphone ED sensor, the four microphones are arranged in a tetrahedron configuration and are used for conventional three-dimensional sensing in an SP system.
The present invention is directed to overcoming the one or more problems or disadvantages associated with the prior art.